Dissecting photosynthetic electron transport and photosystems performance in Jerusalem artichoke (Helianthus tuberosus L.) under salt stress

Jerusalem artichoke (Helianthus tuberosus L.), a vegetable with medical applications, has a strong adaptability to marginal barren land, but the suitability as planting material in saline land remains to be evaluated. This study was envisaged to examine salt tolerance in Jerusalem artichoke from the angle of photosynthetic apparatus stability by dissecting the photosynthetic electron transport process. Potted plants were exposed to salt stress by watering with a nutrient solution supplemented with NaCl. Photosystem I (PSI) and photosystem II (PSII) photoinhibition appeared under salt stress, according to the significant decrease in the maximal photochemical efficiency of PSI (△MR/MR₀) and PSII. Consistently, leaf hydrogen peroxide (H₂O₂) concentration and lipid peroxidation were remarkably elevated after 8 days of salt stress, confirming salt-induced oxidative stress. Besides photoinhibition of the PSII reaction center, the PSII donor side was also impaired under salt stress, as a K step emerged in the prompt chlorophyll transient, but the PSII acceptor side was more vulnerable, considering the decreased probability of an electron movement beyond the primary quinone (ETo/TRo) upon depressed upstream electron donation. The declined performance of entire PSII components inhibited electron inflow to PSI, but severe PSI photoinhibition was not averted. Notably, PSI photoinhibition elevated the excitation pressure of PSII (1-qP) by inhibiting the PSII acceptor side due to the negative and positive correlation of △MR/MR₀ with 1-qP and ETo/TRo, respectively. Furthermore, excessive reduction of PSII acceptors side due to PSI photoinhibition was simulated by applying a specific inhibitor blocking electron transport beyond primary quinone, demonstrating that PSII photoinhibition was actually accelerated by PSI photoinhibition under salt stress. In conclusion, PSII and PSI vulnerabilities were proven in Jerusalem artichoke under salt stress, and PSII inactivation, which was a passive consequence of PSI photoinhibition, hardly helped protect PSI. As a salt-sensitive species, Jerusalem artichoke was recommended to be planted in non-saline marginal land or mild saline land with soil desalination measures.

Effects of Different Planting Densities on Photosynthesis in Maize Determined via Prompt Fluorescence, Delayed Fluorescence and P700 Signals

The mutual shading among individual field-grown maize plants resulting from high planting density inevitably reduces leaf photosynthesis, while regulating the photosynthetic transport chain has a strong impact on photosynthesis. However, the effect of high planting density on the photosynthetic electron transport chain in maize currently remains unclear. In this study, we simultaneously measured prompt chlorophyll a fluorescence (PF), modulated 820 nm reflection (MR) and delayed chlorophyll a fluorescence (DF) in order to investigate the effect of high planting density on the photosynthetic electron transport chain in two maize hybrids widely grown in China. PF transients demonstrated a gradual reduction in their signal amplitude with increasing planting density. In addition, high planting density induced positive J-step and G-bands of the PF transients, reduced the values of PF parameters PI_ABS, RC/CS_O, TR_O/ABS, ET_O/TR_O and RE_O/ET_O, and enhanced ABS/RC and N. MR kinetics showed an increase of their lowest point with increasing high planting density, and thus the values of MR parameters V_PSI and V_PSII-PSI were reduced. The shapes of DF induction and decay curves were changed by high planting density. In addition, high planting density reduced the values of DF parameters I₁, I₂, L₁ and L₂, and enhanced I₂/I₁. These results suggested that high planting density caused harm on multiple components of maize photosynthetic electron transport chain, including an inactivation of PSII RCs, a blocked electron transfer between Q_A and Q_B, a reduction in PSI oxidation and re-reduction activities, and an impaired PSI acceptor side. Moreover, a comparison between PSII and PSI activities demonstrated the greater effect of plant density on the former.